Anisotropic plasma etching methods are known, for example, from German Patent No. 197 06 682 A1 or German Patent No. 42 41 045 C2, in which, in each case, a plasma of neutral radicals and electrically charged particles is produced via a high-density plasma source, the particles being accelerated by a bias voltage source in the direction of a substrate electrode carrying the wafer to be processed. In this context, a directed etching process is achieved by the preferential direction of the incident ions.
Furthermore, high-frequency generators having a carrier frequency of 13.56 MHz are typically used as the bias voltage source that produces the electrical voltage for accelerating the ions from the plasma in the direction of the substrate electrode. In this context, the high-frequency generator is adjusted by an LC network (“matchbox”) to both the impedance of the substrate electrode and the plasma that is in contact with the substrate electrode.
Furthermore, under consideration of a good mask selectivity, i.e., the ratio of the silicon etching rate to the etching speed of the masking layer, it is already known to select the high-frequency power on the substrate electrode to be relatively low to keep the ion-supported mask removal as minimal as possible. Typical power values are between 5 watts and 20 watts, so that the energy of the ions incident on the substrate surface is usually several units of 10 eV.
It is true that such low ion energies are advantageous with respect to the mask selectivity. However, as a result, the incident ions can also have a relatively significant degree of scatter with respect to their direction and can partially deviate from the desired, vertical incidence or can be slightly deflected, i.e., their directionality is low. Such deviations in the directionality of the incident ions then correlates to more difficult profile verification of the produced etching profile. Viewed in terms of the directionality of the ion current, high ion acceleration, i.e., high ion energy, would, therefore, be desirable, which, however, conflicts with the necessary mask selectivity.
Furthermore, charging effects often occur on the boundary layer silicon dielectric when using high-density plasmas having low-energy ion action on a substrate in response to impacting upon an etch stop of dielectrics (buried oxides, lacquer layers, etc.). Profile imperfections in the silicon resulting therefrom are referred to as notching on the dielectric interface.
At the same time, as the ion energy increases, so does the danger of so-called “grass formation” on the etching ground, i.e., the process window for a reliable etching process without grass formation is limited. In this context, “grass formation” refers to the nonuniform etching of the etching ground while forming a plurality of closely adjoining points, which take on the shape of grass.
To achieve this objective, the applications German Patent No. 199 33 842.6 and German Patent No. 199 19 832.2 already proposed pulsing the high-frequency a.c. voltage, which is used for producing the substrate bias, i.e., for producing the substrate electrode power to be coupled into the substrate to be etched, and at the same time, selecting the ion energy to be higher during the high-frequency impulses than for continuous wave operation.
However, during this pulse control operation, it is observed that an effective suppression of the notching is first achieved in response to relatively long interval times of 0.1 ms to 1 ms between the applied high-frequency impulses. If the pulse intervals are shortened to under 0.1 ms, notching occurs more frequently and cannot be suppressed by increasing the peak pulse power and correspondingly shortening the pulse duration.
Moreover, for long interval times of 0.1 ms to 1 ms, the process window for a reliable process, i.e., a grass-free etching ground, narrows in response to the pulse time being shortened with a corresponding increase in the peak pulse power, i.e., the etching process becomes increasingly notch-resistant, but the suppression of a grass-free etching ground becomes increasingly smaller. To date, this requirement for a “notch-resistant” process, therefore, conflicts with a “grass-resistant” process.
In this context, the process window refers to process parameter ranges suitable for implementing an etching process, which is reliable in the explained manner, in particular with respect to process pressure, substrate electrode power, plasma power, and gas flows, as well as, in some instances, the cycle times for alternating etching cycles and passivation cycles.
On the whole, in the known methods under the marginal conditions of a “grass-free” etching ground and a sufficient suppression of “notching,” the employable high-frequency peak pulse powers and, as such, the ion energies, i.e., the directionality of the ion incidence, is, therefore, restricted, thereby resulting, to date, in the process window, i.e., the usable process parameters, being restricted in an undesired manner.
Due to the grass formation, this restriction of the process window has a particularly disruptive effect when high-rate etching processes are to be carried out, since, as such, the range of allowable process pressures is restricted in an upward direction. On the other hand, it is exactly high pressures, high gas flows, and high plasma powers at the inductive source that are advantageous for achieving high etching rates.